It is important for studies regarding human stem cells to conduct measurement in vivo. Thus, such studies are carried out based on xenogeneic transplantation using immunodeficient rodents or sheep fetuses (Flake, A. W. et al., 1986. Science 233: 776-778.). In 1988, McCune et al. have reported Scid-hu assay (Science 241: 1632-1639 (1988)). This assay constituted a first case where human cells were detected in a CB17/SCID mouse. Thereafter, as recipients into which human hematopoietic cells are to be transplanted, many types of immunodeficient mice have been used. Examples of such immunodeficient mice may include NOD/SCID (Pflumio, F. et al., 1996. Blood 88:3731-3740.), NOD/RAG-1null (Shultz, L. D. et al., 2000. Journal of Immunology 164:2496-2507.), beige/nude/scid (Dao, M. A., and J. A. Nolta. 1998. International Journal of Molecular Medicine 1:257-264.), and NOD/SCID/β2Mnull (Kollet, O. et al., 2000. Blood 95: 3102-3105.).
However, a majority of mice used as recipients for transplantation of stem cells in xenogeneic animals are adult mice with an age of 8 to 12 weeks old. In addition, in the case of using common SCID adult mice, in order to maintain a graft for a long period of time, administration of extrinsic cytokine is necessary. Moreover, it is difficult to allow T cells to differentiate from precursor cells (Ito, M. et al., 2002. Blood 100:3175-3182.).